1. Field of the Invention
The present invention relates to a liquid droplet ejecting type image forming apparatus and an atmospheric air opening method of a sub tank of a liquid droplet ejecting type recording head.
2. Description of the Related Art
In a liquid droplet ejecting type image forming apparatus, for example, an inkjet type image forming apparatus, a head unit is provided with a sub tank, and an amount of ink that has been consumed at a recording head is supplied to the sub tank from a main tank. For example, as described in patent document 1, when air enters a supply path to the sub tank, a failure may occur in the negative pressure in the sub tank for a certain time period. Therefore, a negative pressure recovery operation involving an atmospheric air opening operation is performed. The negative pressure recovery operation involving an atmospheric air opening operation is preferably performed only when air enters the supply path, because ink is consumed by performing these operations. However, in order to perform the negative pressure recovery operation involving an atmospheric air opening operation only when air enters the supply path, it would be necessary to determine whether air has entered the supply path. Thus, in consideration of ink consumption, there are two types of negative pressure recovery operations. One type is a negative pressure recovery operation involving an atmospheric air opening operation, and another type is a negative pressure recovery operation that does not involve an atmospheric air opening operation.
FIG. 9 is an exploded perspective view of a conventional inkjet head unit. As shown in FIG. 9, a head unit 1 includes heads 3 on which sub tanks 4 are mounted, an ink supply mechanism 2 for supplying ink to the sub tanks 4, a carriage 5 carrying the heads 3 corresponding to four colors that moves in a main scanning direction, and a support shaft 6 that supports the carriage 5 such that the carriage 5 is movable in the main scanning direction. The ink supply mechanism 2 is used to supply ink from ink cartridges 2Y, 2M, 2C, and 2K corresponding to the respective colors of YMCK, to the sub tanks 4 corresponding to the respective colors provided above the heads 3. The sub tank 4 maintains a negative pressure required for ejecting ink, and constantly maintains a fixed negative pressure for preventing the ink inside the sub tank 4 from dripping out. A flexible film is adhered to the side surfaces of the sub tank 4. The flexible film expands and contracts in accordance with the pressure inside the sub tank 4, so that the internal volume changes. Accordingly, the ink in the sub tank 4 is consumed. The sub tank 4 has negative pressure until a predetermined amount of ink is consumed, and the sub tank 4 may continue to maintain the negative pressure.
The negative pressure is attained by opening the sub tank 4 to atmospheric air so that atmospheric pressure is attained, and ejecting a predetermined amount of ink. For this purpose, the sub tank 4 is provided with an atmospheric air opening mechanism 7. The atmospheric air opening mechanism 7 of the sub tank 4 mounted on the carriage 5 is activated by pressing a pin 8 with a lever 10 of a solenoid 9. The pin 8 contacts the atmospheric air opening mechanism 7 via a pin hole (through hole) 11 provided in the carriage 5, from a direction perpendicular to the carriage moving direction, to activate the atmospheric air opening mechanism 7.
The negative pressure in the sub tank 4 may not be maintained if gas enters the sub tank 4 as time passes or when the ink cartridges 2Y, 2M, 2C, and 2K are replaced. When the negative pressure is no longer maintained, the negative pressure needs to be attained once again. To attain the negative pressure, a predetermined amount of ink is ejected as described above, and therefore ink is wasted. Therefore, the atmospheric air opening operation is preferably not performed under regular circumstances; the atmospheric air opening operation is preferably performed only when necessary. Accordingly, a separate driving mechanism such as a solenoid shown in FIG. 9 is typically provided. Examples are disclosed in patent document 2 and patent document 3.
Meanwhile, in recent years and continuing, printers are being used by users for versatile purposes. Accordingly, there is a wide variety of paper types being used. Particularly, there is a wide variety of paper types with different thicknesses (plain paper, post cards, cardboard, envelopes, etc.). In order to address the wide variety of paper types, patent document 4 discloses a mechanism for adjusting the gap between the paper sheet and the image forming mechanism (head gap).
When a magnet valve or a solenoid is provided for selecting the atmospheric air opening operation as described above, the number of components increases, the control operation becomes complex, and the apparatus size becomes large. Meanwhile, as described in patent document 4, there is known a mechanism for adjusting the gap between the paper sheet and the image forming mechanism. This mechanism is only used for adjusting the gap.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2007-15153    Patent Document 2: Japanese Patent No. 4155879    Patent Document 3: Japanese Laid-Open Patent Publication No. 2007-125825    Patent Document 4: Japanese Laid-Open Patent Publication No. 2008-179103